In the rubber industries, such as tire industry, carbon black for reinforcing the rubber is mixed with rubber in advance by a wet carbon black master batch method whereby the process of kneading the carbon black into the rubber has been attempted to be simplified and improvements in the dispersibility of the carbon black in the rubber have been attempted (for example, see Japanese Unexamined Patent Publication (Kokai) No. 59-49247 and Japanese Unexamined Patent Publication (Kokai) No. 63-43937).
Rubber compounds for tire use are usually used for applications where repeated deformation is applied. If the tan .delta. of the rubber compound in the high temperature region (for example, at 60.degree. C.) is high, however, the amount of heat generated during use etc. becomes high, which is liable to lead to early breakage of the product such as tire. Therefore there has been a need for lowering the tan .delta. in the high temperature region, while maintaining the grip on wet roads, the wear resistance, the breakage strength, and other performances. For example, in tire compounds, improvement of the fuel consumption, durability, etc. requires that the tan .delta. be lowered in the high temperature region (40.degree. C. to 100.degree. C.), in particular at 60.degree. C., but in general the braking performance (for example, wet grip), which is a contradictory property, falls as well. Thus, it becomes necessary to maintain this property.
In recent years, however, the appearance of superior silane coupling agents has led to the formulation of silica instead of carbon black. Silica has a lower tan .delta. at high temperatures (about 60.degree. C.) compared with carbon black and a higher tan .delta. at low temperatures (about 0.degree. C.), and therefore, when used for example for a rubber composition for a tire tread has the advantage of enabling production of a tire having a low rolling resistance and a high grip.
As this prior art, for example, Japanese Unexamined Patent Publication (Kokai) No. 3-239737 proposes blending a silica filler with an SBR obtained by the solution polymerization method or a blend thereof and a diene rubber. Further, Japanese Unexamined Patent Publication (Kokai) No. 3-252431 proposes the blending of a silica filler and silane coupling agent into an SBR obtained by the solution polymerization method or a blend thereof with a diene rubber. Further, Japanese Unexamined Patent Publication (Kokai) No. 3-253433 proposes the blending of a silica filler and silane coupling agent with solution polymerization BR or SBR having silane modified terminals.
While, according to the above-mentioned proposals the tan .delta. at the high temperature region can be reduced, without lowering the braking performance and other properties, however, the addition of the silica means that the wear resistance is inferior to that when just adding carbon black. Further, silica tends to make a high degree of self agglomeration, and therefore, is not easy to disperse. Thus the problem of inferior processability is caused. Further, silica is nonconductive, therefore, when silica is used alone for tire use, in particular tire tread use, without the use of a sufficient amount (normally about 40 parts by weight) of carbon black, results in insufficient flow of static electricity to the road surface and, as a result, charging and static discharge. The resultant discharge causes noise in the radio and other electronic apparatuses and in some cases causes erroneous operation of the same. Further, the vulcanization rate is slower, and therefore, a vulcanization promoter becomes necessary and the cost becomes higher.
Covering the surface of the pigment etc. with silica etc. to improve the dispersibility and improve the weather resistance has been proposed, for example, in Japanese Examined Patent Publication (Kokoku) No. 50-14254, Japanese Examined Patent Publication (Kokoku) No. 7-30269, etc. but none of these alludes to the deposition of silica on the surface of the carbon black for rubber reinforcement.
On the other hand, when blending silica with a rubber composition, it is difficult to cause the dispersion of the silica during the kneading and therefore a large amount of labor becomes necessary for the kneading. Thus, the preparation of master batches has been desired for silica like carbon black. Silica, however, has a different agglomeration pH (about 4 to 7) than the agglomeration pH region of carbon black (about 2.5 to 3), and therefore, in the past a satisfactory wet type silica master batch has not been able to be obtained.